Semiconductor elements have been improved to have high density and to be highly mounted, along further improvement in quality of electronic devices. It is important to more efficiently radiate heat generated from an electronic part constituting an electronic device because of the aforementioned trends. In order to efficiently radiate heat, a semiconductor element is mounted to a heat sink, such as a radiating fin, and a heat sink plate. As for a thermally conductive sheet, a thermally conductive sheet, in which filler (thermally conductive filler), such as inorganic filler, is dispersed in silicone, has been widely used. Examples of the inorganic filler include alumina, aluminum nitride, and aluminum hydroxide.
Various researches have been conducted on the thermally conductive sheet for the purpose of achieving high thermal conductivity.
For example, it has been attempted to increase a filling rate of the inorganic filler blended in a matrix. If the filling rate of the inorganic filler is increased, however, a flexibility of a resulting thermally conductive sheet may be impaired, or powdering may occur due to the high filling rate of the inorganic filler. Therefore, there is a limitation in a method for increasing the filling rate of the inorganic filler.
Moreover, proposed are techniques for adding boron nitride (BN), flaky particles, such as graphite, or carbon fiber in a matrix (for example, see PTL 1 and PTL 2). These proposed techniques are those utilizing anisotropy of thermal conductivity, which the flaky particles or the like have. In case of carbon fiber, for example, there is thermal conductivity of about 600 W/m·K to about 1,200 W/m·K in a direction of alignment of the fiber. In case of boron nitride, there is thermal conductivity of about 110 W/m·K in the in-plane direction, and about 2 W/m·K in a direction vertical to the in-plane direction. These materials have been known to have anisotropy.
In recent years, high density and highly dense mounting of a semiconductor element are significantly advanced. Therefore, even higher thermal conductivity of a thermally conductive sheet has been required. Moreover, it is desired to achieve high thermal conductivity with a thin sheet.
Use of the carbon fiber can provide a thermally conductive sheet, which realizes both excellent high thermal conductivity and flexibility (for example, see PTL 3). However, use of a thin sheet (for example, the average thickness being 500 μm or less) increases thermal resistance at an interface. Therefore, it has not yet provided a thermally conductive sheet, which realizes both excellent high thermal conductivity that is required recently, and excellent flexibility.
Accordingly, there are currently needs for a thermally conductive sheet, which can achieve both excellent high thermal conductivity, and excellent flexibility with a small thickness, a production method thereof, and a semiconductor device using the thermally conductive sheet.